Dynamically reconfigurable shared scan-in test architecture

ABSTRACT

A low overhead dynamically reconfigurable shared scan-in test architecture is provided. This test architecture advantageously allows for changing scan inputs during the scan operation on a per shift basis. The flexibility of reconfiguring the scan input to scan chain mapping every shift cycle can advantageously reduce both test data volume and test application time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to test architectures for integratedcircuits, and in particular to test architectures that allows changingvalues on the scan configuration signals during the scan operation on aper shift basis.

2. Description of the Related Art

Larger and more complex logic designs in integrated circuits (ICs) leadto demands for more sophisticated testing to ensure fault-freeperformance of those ICs. This testing can represent a significantportion of the design, manufacture, and service cost of ICs. In a simplemodel, testing of an IC can include applying multiple test patterns tothe inputs of a circuit and monitoring its outputs to detect theoccurrence of faults. Fault coverage indicates the efficacy of the testpattern in detecting each fault in a universe of potential faults. Thus,if a set of patterns is able to detect substantially every potentialfault, then fault coverage approaching 100% has been achieved.

To facilitate better faults coverage and minimize test cost, DFT(design-for-test) can be used. In one DFT technique, structures in thelogic design can be used. Specifically, a logic design implemented inthe IC generally includes a plurality of state elements, e.g. sequentialstorage elements like flip-flops. These state elements can be connectedinto scan chains of computed lengths, which vary based on the design. Inone embodiment, all state elements in the design are scannable, i.e.each state element is in a scan chain. The state elements in the scanchains are typically called scan cells. In DFT, each scan chain includesa scan-input pin (also called a scan input herein) and a scan-outputpin, which serve as control and observation nodes during the test mode.

The scan chains are loaded with the test pattern by clocking inpredetermined logic signals through the scan cells. Thus, if each scanchain includes 500 scan cells, then 500 clock cycles are used tocomplete the loading process. Note that, for simplicity, the embodimentsprovided herein describe scan chains of equal length. In actualembodiments, DFT attempts to create, but infrequently achieves, thisgoal. Thus, in actual embodiments, software can compensate for thedifferent scan chain lengths, thereby ensuring that outputs from eachtest pattern are recognized and analyzed accordingly. This methodologyis known to those skilled in the art and therefore is not explained indetail herein.

Typically, the more complex the design, the more flip-flops are includedin the design. Unfortunately, with relatively few inputs and outputs ofthe design that can be used as terminals for the scan chains, the numberof flip-flops per scan chain has increased dramatically. As a result,the time required to operate the scan chains, called herein the testapplication time, has dramatically increased.

FIG. 1A illustrates a standard flow 100 for processing a single scantest pattern. In flow 100, step 101 sets up the scan chain configurationusing flip-flops in the design, thereby identifying the scan cells ofthe scan chain. Step 102 shifts the scan-in values into the active scanchains. Step 103 exits the scan configuration. Step 104 applies stimulusto the test circuit inputs and measures the outputs. Step 105 pulses theclocks to capture the test circuit response in the flip-flops. Step 106sets up the scan chain configuration. Step 107 shifts the scan-outvalues from the active scan chains. Step 108 exits the scanconfiguration.

For clarification of various steps, FIG. 1B illustrates a portion of ageneric design including logic 121 and flip-flops 123. In step 101,multiplexers 122 can be added between logic 121 and flip-flops 123.Using a scan_enable (i.e. a control) signal, multiplexers 122 can beconfigured to allow scan-in values to be shifted into flip-flops 123without going through logic 121 in step 102. In step 103, multiplexers122 can be reconfigured to accept values from logic 121. At this point,stimulus can be applied to the test circuit in step 104. A pulse can beapplied to the clock CLK terminals of flip-flops 123 to capture theresulting values in step 105. In step 106, multiplexers 122 can bereconfigured to shift those resulting values out through the scan chaincomprising flip-flops 123. Step 108 marks the end of processing a singlescan test pattern.

Notably, steps 101, 103-106, and 108 take only one clock period on thetester. However, each shift operation, e.g. steps 102 and 107, take asmany clock periods as the longest scan chain. In a complex design,200,000 flip-flops may be included. Assuming that only 10 scan chainscan be provided, each scan chain would then have 20,000 (200,000/10)flip-flops, thereby requiring 20,000 clock cycles to process a singlescan test pattern. Therefore, irrespective of any optimization achievedby overlapping scan operations of adjacent test patterns, testapplication time is dominated by the scan operation.

To detect a single fault, only a limited number of values of the testpattern may be used for fault detection. In fact, for typical testpatterns, only 2% of the scan-in values may be used to test a fault. Theremainder of the test pattern, i.e. the part of the test pattern notcontributing to fault detection, can be filled with “don't care” values(also called logic X's).

Deterministic automatic test pattern generation (ATPG) can be used togenerate the minimum set of patterns while providing fault coverageclose to 100%. Specifically, in deterministic ATPG, each test pattern isdesigned to test for the maximum number of faults. However, even withthe reduction in test patterns, deterministic ATPG patterns for complexdesigns still require significant storage area in the test-applicationequipment for the large number of patterns that are input directly tothe scan chains as well as for the expected output values from the scanchains.

Therefore, a need arises for a test architecture and method thatsignificantly reduces test data volume and test application time in anarea-efficient manner.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A illustrates a standard flow 100 for processing a single scantest pattern.

FIG. 1B illustrates a portion of a generic design including logic andflip-flops.

FIG. 2 illustrates an exemplary Illinois Scan architecture including Nscan chains and m scan inputs.

FIGS. 3A and 3B illustrate exemplary dependencies for shared scaninputs.

FIG. 4 illustrates an exemplary dynamically reconfigurable sharedscan-in test architecture that allows for changing scan inputs on a pershift basis.

FIGS. 5A-5D illustrate a simplified example of a dynamicallyreconfigurable feature.

FIG. 6 illustrates a technique to create a dynamically reconfigurableshared scan-in test architecture.

FIG. 7A illustrates another exemplary dynamically reconfigurable sharedscan-in test architecture in which the multiplexer preceding one scanchain can be advantageously shared by another scan chain.

FIG. 7B illustrates yet another exemplary dynamically reconfigurableshared scan-in test architecture in which additional multiplexers canallow for the serial configuration of the scan chains.

FIGS. 8A, 8B, 8C, and 8D illustrate tables showing exemplary rotationmappings for three, four, five, and six scan inputs, respectively.

SUMMARY OF THE INVENTION

A low overhead dynamically reconfigurable shared scan-in testarchitecture is provided. This test architecture advantageously allowsfor changing scan inputs during the scan operation on a per shift basis.The flexibility of reconfiguring the scan input to scan chain mappingevery shift cycle can advantageously reduce both test data volume andtest application time.

This re-configurability feature can be implemented using multiplexinglogic, e.g. multiplexers that can selectively connect the scan inputs tothe scan chains. The control signals of this multiplexing determine theselected configuration. These control signals must be independent of theother scan-enable signals of the scan chains (see FIG. 1B). In oneembodiment, each multiplexer can selectively couple a scan input to aplurality of scan chains.

In one embodiment, each configuration can use a number of scan inputsthat is relatively prime to numbers used by other configurations. Ofimportance, to maximize data volume reduction, the configuration usingthe largest scan-in fan-out (i.e. the smallest number of scan inputs)can be used first. If that configuration results in a conflict ofscan-in values, then the number of scan inputs can be increased for thenext configuration, thereby decreasing the probability of conflict. Inaccordance with one use of the dynamically reconfigurable shared scan-intest architecture, successive configurations can be used until noconflict occurs.

In one embodiment, successive configurations can use 2, 3, 5, 7, 11 . .. scan inputs. Thus, if three configurations are to be constructed with12 scan chains, then the following scan input mapping can be generatedwherein the scan chains can be numbered sc₁,sc₂,Λ,sc₁₂ and the scaninputs can be numbered si₁,si₂,Λ,si_(m). The first configuration can use2 scan inputs. In this case, all the odd numbered scan chains can beconnected to scan input si₁ and all the even numbered scan chains can beconnected to scan input si₂. The second configuration uses 3 scaninputs, where the scan chains connected to scan input si₁ aresc₁,sc₄,sc₇,sc₁₀, the scan chains connected to scan input si₂ aresc₂,sc₅,sc₈,sc₁₁ and the scan chains connected to scan input si₃ aresc₃,sc₆,sc₉,sc₁₂. The third configuration can use 5 scan inputs, whereinscan input si₁ is connected to scan chains sc₁,sc₆,sc₁₁, scan input si₂is connected to scan chains sc₂,sc₇,sc₁₂, scan input si₃ is connected tosc₃,sc₈, scan input si₄ is connected to scan chains sc₄,sc₉, and scaninput si₅ is connected to scan chains sc₅,sc₁₀.

Advantageously, a conflict that occurs within one configuration (e.g.the configuration with 2 scan inputs) can be resolved by using the nextconfiguration (i.e. the configuration with 3 scan inputs). Because thenumber of scan inputs can be dynamically changed on a per shift basis,conflicts in a test pattern can be resolved using a small number ofconfigurations (e.g. 3 configurations). Thus, a dynamicallyreconfigurable scan-in technique can quickly and efficiently match theneeds of a test pattern, thereby significantly improving test datavolume and test application time compared to a static-onlyconfiguration.

In another embodiment, scan inputs can be mapped to scan chains using arotation method. In this case, the ordering of the scan inputs forsuccessive configurations can change after each application of a scaninput set (e.g. scan inputs 0, 1, and 2). For example, a firstconfiguration could provide a rotation of zero (e.g. 012, 012, 012),whereas a second configuration could provide a rotation of one (e.g.012, 120, 201).

In a method of performing a scan operation on a test design using thedynamically reconfigurable shared scan-in test architecture is alsoprovided. In this method, a first set of scan inputs can be mapped to afirst plurality of scan chains (i.e. a first configuration) on a firstshift cycle. If the first configuration results in at least oneconflict, then a second set of scan inputs can be mapped to a secondplurality of scan chains (i.e. a second configuration) on a second shiftcycle. Notably, a membership of scan cells within each scan chain is thesame for the first and second configurations. Additional configurationscan be used as necessary to eliminate conflicts.

A method of forming a dynamically reconfigurable shared scan-in testarchitecture is also provided. This method, which can be performed usinga computer-implemented software program, includes creating scan chainswithout association to scan inputs using cones of influence (i.e.determining scan cell membership to a scan chain) and then creating anassociation between the scan chains and a variable number of scan inputs(i.e. determining scan input mapping). Creating the association betweenthe scan chains and the variable number of scan inputs can includedefining a plurality of configurations. Defining the plurality ofconfigurations can include using a predetermined number of scan inputsfor each configuration. In one embodiment, successive configurations canuse 2, 3, 5, 7, 11 . . . scan inputs. In another embodiment, mapping ofscan inputs to scan chains can be done using a rotation method.Advantageously, both scan cell membership and scan input mapping can bedetermined without any design analysis.

DETAILED DESCRIPTION OF THE FIGURES

To detect a single fault, only a limited number of scan-in values of thetest pattern may be used for fault detection. The remainder of the testpattern includes “don't care” values. Taking into account the largenumber of don't care values in any pattern, some test architecturesattempt to share scan-in values to decrease data volume. For example, atest architecture called Illinois Scan uses a limited number of commonscan inputs to allow for the don't cares of the scan cells to be filledwith the same values as those in other scan cells.

FIG. 2 illustrates an exemplary Illinois Scan architecture 200 includingN scan chains and m scan inputs. In architecture 200, each scan-in valueis provided to N/m scan chains. Because m represents a number consistentwith conventional scan architectures, the shared scan-in values allowfor many shorter scan chains compared to conventional scanarchitectures. After the scan-in values are processed through testdesign 201, a MISR 202 can be used to compact the responses. In oneembodiment, MISR 202 can include a compactor 203 and a plurality offlip-flops 204.

In architecture 200, the scan chains sharing the same scan-in value havecertain dependencies. For example, FIGS. 3A and 3B illustrate exemplarydependencies. In FIG. 3A, a scan-in value is shared by four scan chains300A, 300B, 300C, and 300D (a MISR 305 is shown for reference). In thissimplified embodiment, each scan chain includes four scan cells (e.g.flip-flops), which are represented by squares. Scan cells having thesame fill pattern have been provided the same scan-in value. Thus, eachof scan cell sets 301, 302, 303, 304 stores the same value in a scanoperation (wherein, for example, a scan-in value stored by scan cell set301 is shifted to scan cell set 302 after a clock is applied). FIG. 3Billustrates a more aggressive sharing of a scan-in value wherein each ofscan cell sets 311 and 312 includes eight scan cells. Therefore, in thisconfiguration, a scan-in value is shared by eight scan chains.

Logically, if a single scan-in value is provided to only a few scanchains, then the probability of a scan cell requiring an opposite valueto that of the scan-in value is very low. In contrast, if a singlescan-in value is provided to many scan chains, then the probability of ascan cell requiring an opposite value to that of the scan-in value isvery high. Therefore, increasing the sharing (i.e. fan-out) of a scan-invalue can decrease data volume at the risk of increasing conflict.

Test architectures must also take into account test application time.For example, in a complex design, if only a few scan inputs areavailable (and thus a corresponding number of scan chains), then thenumber of scan cells in each scan chain can be great, therebyundesirably increasing test application time. In contrast, if many scaninputs (and thus scan chains) are available, then the number of scancells in such scan chains can be decreased, thereby reducing testapplication time. Therefore, for example, the scan-in configuration ofFIG. 3B could significantly reduce test application time compared to thescan-in configuration of FIG. 3A.

In a shared scan-in architecture, both test data volume and testapplication time can be reduced only when the dependencies do notconflict with test pattern requirements. In other words, if the testpattern does not require different scan-in values for a scan cell set,then no conflict exists. If a test pattern can use a shared scan-inarchitecture, then the test data volume as well as the test applicationtime can be reduced by a factor of Z, wherein Z corresponds to thenumber of scan chains receiving a scan-in value. For example, in FIG.3A, both the test data volume and test application time can be reducedby a factor of four if no conflict exists.

Conflicts in the scan cell set require a shared scan-in architecture toresort to other scan methods. For example, referring back to FIG. 2, aplurality of multiplexers 205 can be used to receive both the scan-invalue as well as an output of a preceding scan chain, if present. Inthis configuration, if a test pattern results in a conflict within ascan cell set, then the test pattern can be applied through atraditional serial scan chain formed by scan chains 1-N usingmultiplexers 205. Unfortunately, after a serial scan chain configurationis used, architecture 200 cannot revert back to the shared scan-inconfiguration. Notably, the use of a serial scan chain eliminates anyfurther test data volume or test application time reduction.

A dynamically reconfigurable shared scan-in test architecture canadvantageously reduce conflicts while still reducing test data volumeand test application time. In accordance with one feature of thisdynamically reconfigurable shared scan-in test architecture,multiplexing logic (e.g. multiplexers) can be used at the beginning ofthe scan chains to allow for multiple alliances between the scan inputsand the scan chains.

FIG. 4 illustrates an exemplary dynamically reconfigurable sharedscan-in test architecture 400 including a plurality of scan chains 1-Nformed in a test design 401 (a MISR 402 is shown for reference). Inarchitecture 400, each of a plurality of multiplexers 403 can be coupledto receive the signals on scan inputs i₁-i_(m). Note that in thisembodiment, multiplexers 403 can also be coupled to receive the invertedsignals provided on scan inputs i₁-i_(m).

Based on their control signals, multiplexers 403 select N scan-in valuesto be provided to scan chains 1-N.

Specifically, using one setting of the multiplexer control signals, ascan chain can be connected to a particular scan input and in anothersetting the same scan chain can be connected to a different scan input.Note that multiple scan chains can be connected to the same scaninput—and hence would get the same scan-in value.

The multiplexer control can be kept constant during the application of atest. Alternatively, multiplexer control can be dynamically changedwhile applying the same test. Advantageously, changing the multiplexercontrol during the scan operation provides dynamism in the testarchitecture that is the equivalent of a larger number of staticreconfigurations. This dynamically reconfigurable feature dramaticallyincreases the probability that a test pattern can be applied withoutconflict. FIGS. 5A-5D illustrate a simplified example of thisdynamically reconfigurable feature.

FIG. 5A illustrates a test design 500 that includes 16 scan cellsconfigured into 4 scan chains 504A-504D (scan-out logic 505 is shown forreference). These scan chains are connectable to two scan inputs 501 and502 through multiplexers 503. In this simplified example, each ofmultiplexers 503 is a 2-to-1 multiplexer having a “0” and a “1” inputterminal, wherein a logic 0 control signal selects the value on the “0”input terminal and a logic 1 control signal selects the value on the “1”input terminal. In the embodiment shown in FIG. 5A, scan input 501 isconnectable to scan chains 504A, 504B, and 504C. Scan input 502 isconnectable to scan chains 504B, 504C, and 504D.

When the multiplexer control signal is static for the scan only twoconfigurations are possible. As used herein, the term “configuration”refers to a membership of scan chains to scan inputs. For example, FIG.5B shows the configuration when the multiplexer control signal is “0000”during scan. The scan cells of scan chains 504A-504D have fill patternsto show their relationship to the scan inputs. In this configuration,scan chains 504A and 504B are connected to the scan input 501 whereasscan chains 504C and 504D are connected to scan input 502. Similarly,FIG. 5C shows the configuration when the multiplexer control signal is“1111” during scan. In this configuration, scan chains 504A and 504C areconnected to scan input 501 and scan chains 504B and 504D are connectedto scan input 502.

Of importance, when the multiplexer control signal is changed duringscan, significantly more configurations are possible as each shift couldtake on one of the available static configurations. For example, FIG. 5Dshows the scan-in fan-out when the multiplexer control signal is “0011”during scan (i.e. the multiplexer control signals is logic 1 for thefirst two shift cycles and then logic 0 for the remaining two shiftcycles).

Note that many test patterns could have conflicts in certain shiftlocations such that neither of the static configurations shown in FIGS.5B and 5C could be used. However, these same test patterns having thesame conflicts in the same shift locations could be applied in thedynamic configuration shown in FIG. 5D. The ability to have many moreconfigurations at the expense of some test data on the multiplexercontrol allows the dynamically reconfigurable shared scan-in testarchitecture to provide a very efficient platform for test patterns.

Note that dynamic reconfiguration requires that the timing of themultiplexer control signal be adjusted to match the shift operation.Additionally, the multiplexer control signals can be separate from theother scan-enable signals of the scan chains. Because shifting isnormally done at a much slower speed than the operation of the design,this adjustment can be easily achievable.

FIG. 6 illustrates a technique to create a dynamically reconfigurableshared scan-in test architecture. This technique can be advantageouslyimplemented using a computer (or any other suitable device) running asoftware program. In step 601, the scan chains can be created withoutany association to scan inputs. In step 602, the association between thescan chains and the scan inputs can be created for each configuration.These steps will now be explained in further detail.

In step 601, the goal is to construct scan chains such that the numberof potential conflicts between scan chains is minimized. A potentialconflict is defined to exist between any pair of scan cells that belongto the same “cone” of influence and are placed in two different scanchains. A cone of influence (hereinafter cone) refers to flip-flops andthe logic that those flip-flops drive (e.g. flip-flops 123 and logic 121of FIG. 1B would be considered in the same cone of influence).

Tests for faults in a cone require values from the scan cells drivingthe cone. Scan values required from scan cells in the same scan chaincan never conflict. The potential conflict becomes a real conflict whenthe event occurs that satisfies all of the following additionalcriteria.

-   -   1. Values needed in two scan cells are not compatible (a logic 0        and a logic 1).    -   2. The two scan cells are in the same shift position relative to        the scan-in of the chains.    -   3. The two scan cells are in scan chains that are sharing the        same scan-in.

Thus, the cone can be used as a simple mechanism of constructing thescan chains. As previously described, if the test design has F scancells and N scan chains are used, then the length of each scan chain isL, where L=F/N. Topological cones can be constructed for everyobservable point of the test design and sorted by size. Note thatsorting by size provides lower possibility of conflicts with the scanchain to scan input assignment scheme used.

Starting with the inputs of each cone in the list created, the first Lunassigned scan cells encountered can be assigned to a partition for thecreation of a scan chain. The following L cells can be assigned to thenext partition for the creation of another scan chain. This process canbe continued until all scan cells are assigned to some scan chainpartition. (Note that other types of analysis, including standard DFTanalysis that considers routing and other constraints, can be used withor in lieu of the cone technique to construct the scan chains.)

Using this selection technique, most scan cells in a given cone willprobably be either in the same scan chain or in scan chains immediatelybefore/after the scan chain. Therefore, a majority of the scan cellsthat have values required by a test pattern are either in the same scanchain or in adjacent scan chains. (Note that the overlapping of conesmay cause scan cells within a cone to not reside in the same or adjacentscan chains with scan cells in the same cone.) Scan chain membershipbased on cones can facilitate mappings that minimize conflicts.

Step 602 includes creating a mapping between the scan chains and theavailable scan inputs. While every scan chain could be selectivelycoupled to all the scan inputs, another mapping can facilitateminimizing analysis of the test design. Specifically, to maximize datavolume reduction in one embodiment, the configuration using the largestscan-in fan-out (i.e. the smallest number of scan inputs) can be usedfirst. If that configuration results in a conflict, then the number ofscan inputs can be increased for the next configuration, therebydecreasing the probability of conflict. In other words, the fan-out isdecreased only if necessary to avoid conflict. Therefore, in accordancewith one use of the dynamically reconfigurable shared scan-in testarchitecture, various configurations can be used until no conflictoccurs.

In one embodiment, each configuration could use a number of scan inputsthat is relatively prime to the numbers used by other configurations.Thus, successive configurations could use 2, 3, 5, 7, 11 . . . scaninputs. Advantageously, using this succession of scan inputs and thepreviously established membership technique for scan cells means that aconflict occurring in one configuration can be eliminated by the nextconfiguration.

In other words, a desired number of configurations can be determined byselecting configurations beginning with m=2 (wherein m is the number ofscan inputs available for use in the configuration) and using relativelyprime numbers for m. For any given configuration, the scan chains areassigned to the available scan inputs of the configuration such thatevery m^(th) scan chain is connected to the same scan input.

For example, assume that three configurations of the common scan-inarchitecture are to be constructed with 12 scan chains. The scan chainscan be numbered sc₁, sc₂,Λ,sc₁₂ whereas the scan inputs can be numberedsi₁, si₂,Λ,si_(m). The first configuration uses m=2 scan inputs. Thus,all the odd numbered scan chains are connected to scan input si₁ and allthe even numbered scan chains are connected to scan input si₂. Thesecond configuration uses m=3, where the scan chains connected to scaninput si₁ are sc₁,sc₄,sc₇,sc₁₀, the scan chains connected to scan inputsi₂ are sc₂,sc₅,sc₈,sc₁₁ and the scan chains connected to scan input si₃are sc₃,sc₆,sc₉,sc₁₂. The third configuration has five scan inputs,wherein scan input si₁ is connected to scan chains sc₁,sc₆,sc₁₁, scaninput si₂ is connected to scan chains sc₂,sc₇,sc₁₂, scan input si₃ isconnected to sc₃,sc₈, scan input si₄ is connected to scan chainssc₄,sc₉, and scan input si₅ is connected to scan chains sc₅,sc₁₀.

Notably, the mapping of the configurations repeat after the least commonmultiple of the configurations is achieved. Therefore, for example, ifthree configurations are defined, then there would be 2*3*5=30 uniquemappings of scan inputs to scan chains. In this case, each mapping canuse one 3-1 multiplexer (or logic that is equal to 6 two-input gates),thereby creating a total overhead of 180 gates for the input side,regardless of the number of scan chains.

In this embodiment, the multiplexer preceding one scan chain can beadvantageously shared by another scan chain. For example and referringto FIG. 7A (which shows 60 scan chains 701-760), assuming that m=5,which has 30 unique mappings, then scan chain 731 will receive the samescan input value as scan chain 701 (likewise scan chains 732 and 702receive the same scan input value, and scan chains 760 and 730 receivethe same scan input value). That is, the selective coupling of scaninputs to scan chains will repeat every 30 scan chains. Therefore, onecan use N multiplexers provided for N scan chains or the number ofmultiplexers based on the number of unique mappings associated with thenumber of defined configurations (e.g. 30).

Note that, in other embodiments, additional multiplexers can be providedto allow for a default serial configuration of the scan chains. Forexample, referring to FIG. 7B, if a predetermined number ofconfigurations (e.g. 3) have been tried for a test design and stillgenerate at least one conflict, then the scan chains can be reconfiguredinto a single serial scan chain using multiplexers 780, sub-multiplexers781, and serial interconnect (shown as dashed lines) 782. Note thatmultiplexers 780 can be implemented with 4-to-1 multiplexers whereasmultiplexers 781 can be implemented with 2-to-1 multiplexers.

The output side (not shown) can be implemented using a non-redundantXORing of the scan chains to the available scan outputs. In thatconfiguration, the overhead would be one XOR per scan chain, which wouldbe equivalent to 3 two-input gates. In another embodiment, the outputside can be implemented using a MISR, which would result in a differentarea overhead.

The dynamically reconfigurable shared scan-in test architecture cansignificantly reduce test data volume, as will be demonstrated byreference to exemplary test designs (i.e. designs A, B, and C), whichare described below in Table 1.

TABLE 1 Exemplary Circuits To Show DVR Scan Cone size Design GatesFaults cells Max. inputs Max. gates A  230k  481k  9700 432 1887 B  390k 554k 12500 282 916 C 1083k 2740k 69000 264 5454

To maximize test data reduction in these designs, static configurationscan be used first followed by dynamic configurations, as necessary. Todetermine test data volume, computations can be performed by first usingthe static configurations with the least number of scan pins (i.e. asmall m to large m). Because using fewer scan pins (i.e. a smaller m)implies less test data volume, performing the computation using thispriority highlights the test data volume reduction at the expense ofsome test application time. (In contrast, using all scan pins all thetime would improve the test application time at the expense of test datavolume.) An ATPG execution could bias its utilization of configurationsto static configurations over dynamic configurations or, alternatively,immediately utilize the dynamic configurations.

The data volume reduction (DVR) can be calculated as follows. Ingeneral, the DVR can be represented by:DVR=DV _(ATPG) /DV _(NEW)DV _(ATPG)=TestPatterns*ScanChains*MaxChainLength(L)DV _(NEW) =DV _(STATIC1) +DV _(STATIC2) + . . . +DV _(STATICM) +DV_(DYNAMIC)DV _(STATICi)=Patterns in configurationi*[ScanPinsUsed(m)*MaximumChainLength(L)+UnusedScanPins]DV _(DYNAMIC)=Dynamicpatterns*(ScanPinsUsed(m)+ControlPins(t))*MaximumChainLength(L)

In the static configurations, m bits of data can be loaded L times perpatterns and each unused scan pin can be specified once in each vector.For dynamic testing, all the scan pins can be loaded L times for eachpattern. Tables 2A and 2B show the results of these computations for thedesigns provided in Table 1 as well as for three larger ISCAS benchmarkcircuits (i.e. s13207, s38417, and s38584).

TABLE 2A Computations Using Illinois Scan Architecture. Regular IllinoisScan ATPG Broad. Serial DVR = Design Chains m t L Patterns Pat. Pat.TATR A 487 7 2 20 966 2138 218 4.27 B 516 7 2 26 748 1452 438 1.69 C 5377 2 135 2361 3185 418 5.49 S13207 80 7 2 11 149 157 78 1.80 S38417 129 52 14 137 406 60 2.07 S38584 139 7 2 13 230 286 142 1.57

TABLE 3B Computations Using Dynamically Reconfigurable DynamicallyReconfigurable Architecture Design Chains m t L Patterns Max m DVR TATRA 487 7 2 20 2910 7 64.42 18.04 B 516 7 2 26 2767 7 48.00 15.48 C 537 72 135 3715 7 138.6 35.82 S13207 80 7 2 11 305 7 9.30 4.31 S38417 129 5 214 731 5 9.14 3.43 S38584 139 7 2 13 595 7 13.56 2.97

Architecture

As shown by Tables 2A and 2B, the dynamically reconfigurable testarchitecture can overcome the dependencies caused by the common scan-infor significant benefits over the non-reconfigurable Illinoisarchitecture. Max m=7 means that 4 configurations were implemented inthe architecture (i.e. m=2, 3, 5 and 7). Similarly, Max m=5 means 3reconfigurations were implemented and Max m=11 means 5 configurationswere implemented.

The test application time (TAT) depends on the length of the longestscan chain during regular ATPG. In general, the TAT can be computed by:TAT=TAT _(ATPG) /TAT _(NEW)TAT _(ATPG)=TestPatterns*MaximumChainLengthTAT _(NEW) =TAT _(STATIC1) + . . . +TAT ^(STATICM) +TAT _(DYNAMIC)TAT ^(STATICi)=PatternsInConfiguration i*MaximumChainLength(L)TAT _(DYNAMIC)=DynamicPatterns*MaximumChainLength(L)

Table 3 compares using a static-only configuration and a combined staticand dynamic configuration in the dynamically reconfigurable testarchitecture.

TABLE 3 Static and Dynamic Testing Static Only Static + Dynamic PinsTotal Dynamic Pins Design Patterns Configurations (m) Patterns PatternsConfigurations (m) A 2922 6 13 2910 22 4 7 B 2780 5 11 2767 34 4 7 C3712 5 11 3715 25 4 7 s13207 305 5 11 305 6 4 7 s38417 735 6 13 731 16 35 s38584 562 6 13 595 45 4 7

To obtain these results, the number of configurations needed to applyall the patterns through the shared scan-in was determined. Then anexecution was performed with fewer static configurations and a clean-uppass using the dynamic configuration to apply all the remainingpatterns. As Table 3 indicates, the combined static and dynamicconfigurations significantly reduce the number of input pins that areneeded compared to using static only configurations. Additionally, thecombined static and dynamic configurations require fewer configurationscompared to using static only configurations.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the figures, it is to be understoodthat the invention is not limited to those precise embodiments. They arenot intended to be exhaustive or to limit the invention to the preciseforms disclosed. For example, the multiplexers used in thereconfigurable shared scan-in test architecture can receive the samescan enable (i.e. control) signal or different scan enable signals.

Moreover, on the input side of the dynamically reconfigurable sharedscan-in test architecture, many different mappings of the scan inputs toscan chain segments could exist. For example, in one embodiment, scaninputs can be mapped to scan chains using a rotation method. FIG. 8Aillustrates a table showing exemplary rotation mappings for three scaninputs 0, 1, and 2, wherein each row of the table represents aconfiguration and each column of the table represents a scan chain.

Specifically, the top row, which represents a first configuration, has azero rotation. That is, the ordering of the scan inputs does not changeafter each application of a scan input set (e.g. scan inputs 0, 1, and2). Hence, if nine scan chains were provided (as shown in the table ofFIG. 8A), the first, fourth, and seventh scan chains would receive scaninput 0. The second, fifth, and eighth scan chains would receive scaninput 1. Finally, the third, sixth, and ninth scan chains would receivescan input 2.

The middle row, which represents a second configuration, has a rotationof one. That is, the ordering of the scan inputs changes by one aftereach application of a scan input set. In this case, the first, sixth,and eighth scan chains would receive scan input 0. The second, fourth,and ninth scan chains would receive scan input 1. Finally, the third,fifth, and seventh scan chains would receive scan input 2.

The bottom row, which represents a third configuration, has a rotationof two. That is, the ordering of the scan inputs changes by two afterapplication of a scan input set. In this case, the first, fifth, andninth scan chains would receive scan input 0. The second, sixth, andseventh scan chains would receive scan input 1. Finally, the third,fourth, and eighth scan chains would receive scan input 2.

In accordance with one aspect of the invention, rotation mapping can beapplied to any number of scan inputs. For example, FIGS. 8B, 8C, and 8Dillustrate tables showing exemplary rotation mappings for four, five,and six scan inputs, respectively. Once again, each row of these tablescan represent a configuration having a different rotation (e.g. thebottom row of the table in FIG. 8D has a rotation of four).

Many modifications and variations of the reconfigurable shared scan-intest architecture will be apparent. Accordingly, it is intended that thescope of the invention be defined by the following claims and theirequivalents.

1. A method of performing a dynamic scan operation including a pluralityof shift cycles on a test design, the method including: on a first shiftcycle, using a first configuration for coupling a first set of scaninputs to a first plurality of scan chains; and on a second shift cycle,using a second configuration for coupling a second set of scan inputs toa second plurality of scan chains; analyzing scan outputs of the firstand second pluralities of scan chains based on the first and second setsof scan inputs, respectively; and generating results based on analyzedscan outputs, the results identifying any occurrence of a fault, whereindynamically using the first and second shift cycles during a scanincreases a probability that a test pattern can be applied to the testdesign without conflict.
 2. The method of claim 1, wherein the firstplurality of scan chains is greater than the second plurality of scanchains.
 3. The method of claim 1, wherein the first configurationprovides greater fan-out of a scan-in value than the secondconfiguration.
 4. The method of claim 1, wherein a membership of scancells within each scan chain is the same for the first and secondconfigurations.
 5. The method of claim 1, wherein each configurationuses a number of scan inputs that is relatively prime to numbers used byother configurations.
 6. The method of claim 1, wherein successiveconfigurations use 2, 3, 5, 7, 11 . . . scan inputs.
 7. The method ofclaim 1, wherein successive configurations use rotated scan inputs.